1. Field of the Invention
The present invention relates to a charging technology of batteries, and more particularly to an enhancement of its reliability and versatility.
2. Description of the Prior Art
A conventional battery charger is shown in FIG. 1. The portion enclosed by a single-dot chain line is a battery 2. Inside the battery 2, there are laminated cells 4 and a thermostat 6. Usually, one cell 4 has a nominal output of 1.2 V, and the battery voltage is determined by the number of the cells 4 that are laminated. Generally, batteries in a range of nominal voltage of 2.4 V to 12 V are widely used. The thermostat 6 is designed to detect the completion of charging by elevation of temperature.
A commercial alternating-current input of 100 V (or 200 V) is converted to a value suited to charging the battery 2 by means of a transformer 8. The alternating-current output the transformer 8 is rectified by diodes 10 and 12, and is applied to the battery 2. As the battery 2 is being charged, the temperature of the battery 2 goes up. The thermostat 6 is preset to open at a temperature near the end of charging. When the thermostat 6 is open, a detecting circuit 14 turns off an SCR 16, thereby stopping charging.
The conventional battery charger, however, exhibits the following problems.
First, the output voltage of the transformer 8 must be varied depending on the nominal voltage of the battery to be charged. This is because the battery 2 may be damaged or may not be charged if the battery is charged at an improper voltage. Accordingly, for every different nominal voltage of the batteries, an exclusive charger must be prepared, and the terminal and other shapes must be changed so as to prohibit use in improper combinations. As a result, the manufacturing cost is increased.
Second, the charger is easily influenced by the fluctuations of the supply voltage (about 10% in the commercial power supply). That is, as the supply voltage becomes higher, the charging current increases. As a result, the heat generation per unit time of the transformer 8 of the charger or the diodes 10, 12 or SCR 16 increases, which may lead to their deterioration or destruction.
Japanese unexamined patent publication HEI 1-186130 discloses a battery charger where above problems are solved. FIG. 2 shows a circuit diagram of the battery charger. In this battery charger, the value of the charging current flowing to the battery 2 is detected by a resistance R5 and then integrated by an integrating circuit 24. That is, an effective value of the charging current flowing to the battery 2 is obtained from the integrating circuit 24. A microcomputer 100 receives the output of the integrating circuit 24. A/D converter 100e converts the integrated charging current level into digital value. Reference value of the charging current has been written in a ROM 100b of the microcomputer 100. The microcomputer 100 controls the conduction angle of SCR 10 and SCR 12 through an output port 100H and a drive circuit 60 depending on the output value of the integrating circuit 24 and the reference value stored in the ROM 100b, to equalize the effective value of charging current with the reference value. That is, when the output value of the integrating circuit 24 is larger than the reference value stored in ROM 100b, the microcomputer 100 reduces the conduction angle of SCR 10 and SCR 12. When the output value of the integrating circuit 24 is smaller than the reference value stored in ROM 100b, the microcomputer 100 increases the conduction angle of SCR 10 and SCR 12.
The battery charger shown in FIG. 2 can be used commonly regardless of the nominal battery voltage and is not affected by the fluctuations of the supply voltage. It is necessary, however, for controlling the conduction angle of the SCR's to use the microcomputer 100, which makes results in a complicated structure and high-cost of the battery charger. Further, there is the possibility of unusual program running when the microcomputer is used in a noisy or high temperature environment. Accordingly, it is desired to present a reliable battery charger which is capable of controlling the conduction angle without the microcomputer.